Aspects of the present invention relate to an enhanced process to produce a thermofertiliser from potassium-bearing minerals, more specifically silicate minerals and phosphate mineral using a thermal treatment process.
It is generally known in the art that potassium can be extracted from potassium-bearing minerals, like slate ores, using either thermal or hydrometallurgical methods, or combinations of both. There are studies about the dissolution of potassium from potassium minerals (feldspar, mica) and from glauconite in inorganic acids. Fogler and Lung (1975), among other researchers, have studied the dissolution of potassium feldspar in HF and HCl acid mixtures.
Preliminary studies have been conducted by Eichler (1983), from Universidade Federal de Lavras, in the state of Minas Gerais, Brazil, on assayed mixtures with 50% of potash-bearing ore and 50% limestone. The researchers concluded that for use as a potash source, the minerals have to be calcinated together with a source of limestone. Accordingly, these mixtures were calcined at about 1,100° C. during lab tests, and assayed for agronomic use. The results conducted in corn culture show that the efficiency of potassium absorption was equivalent to traditional potash fertilizer (KCl). The main focus of the above study was to test the feasibility of the use of a specific silicate mineral, called Verdete, as a potash source for the production of fertilizers, and the feasibility of this technique was confirmed.
The above results from Eichler (1983), related to the agronomic assay indicate that, even when the final product is a compound that has low solubility in water and a relatively high solubility in citric acid, the agronomic efficiency of the mineral is satisfactory, particularly when compared to a mineral coming from a conventional potash source such as KCl, which is completely soluble in water.
U.S. Pat. No. 4,334,908 describes the production of a vitreous fertilizer that was obtained from raw materials having high water solubility (NH4H2PO4, KH2PO4 and CaHPO4), where these raw materials were mixed with feldspar or granite, the feldspar and granite being added in order to produce potassium (K) and phosphorous (P), which are insoluble in water. This patent uses raw materials that are expensive and that must be processed to decrease their water solubility. This patent has an objective to reduce the water solubility of compounds which raw materials have high water solubility. Reducing the water solubility is generally very expensive. The process described in this patent starts with the use of pure raw materials that already have high solubility in water, and aims to render these materials insoluble in water and to promote a slow release of their nutrients from the fertilizer produced from the materials. In contrast to the above patent, various aspects of the process described in the present invention propose a different product and starts with materials that are insoluble in water to later become soluble in citric acid in order to avoid nutrient losses in the final thermofertiliser produced.
U.S. Pat. No. 4,313,753 describes a process where a mixture of a potassium source, such as caustic potash or potassium carbonate, and a silicon source, such as fly ash, is dried and calcined at a temperature of about 600° C. to about 1,000° C. According to this process, the silicon content in the fly ash reacts with the potassium resulting in the potassium product having high solubility in citric acid. As in the previously discussed patent, this patent has for objective to reduce the water solubility of raw materials that originally have high water solubility. The complexity of equipment and process to be used in the above patent can result in a very costly process.
The roast-leach method for extraction of potassium has also been extensively studied over the years. Mazumber, Sharma and Rao (1993) disclosed a process for the extraction of potassium from glauconite sandstone. Mazumber, Sharma and Rao described the roast kinetics with calcium chloride, where after a roasting step, potassium is recovered from potassium chloride by leaching using distilled water. Further developments in this process were made, but industrial applications using the process and concept of Mazumber, Sharma and Rao is very costly and has a low efficiency. Direct acid leaching is also known as an alternative process for the roast and leaching treatment for obtaining products containing high grade of a mineral such as potash.
It is also known in the art that the use of a sintering process for iron ore and also for a mixture of phosphate concentrate and slag, the latter one to produce a phosphate fertilizer. Such arrangement is describe in the patent document BRPI 9501458-6, filed by Usiminas in Brazil. This application uses aciaria slag in sintering processes for production of agricultural thermo-phosphate fertilizers by mixing the slag with phosphates, lime and/or dolomite, sodium salts, solid carbonaceous fuel and water in a mixer. The focus of this process is to overcome the economic and environmental problems of storage of slag.
Although being largely known, fusion processes or independent potassium extraction techniques such as calcination followed by leaching are generally considered to result in high costs and to have a low efficiency in many cases.